Twenty-two species of Hu IFN-alpha were derived from sendai virus induced Namalwa cells. We found that species O exhibits the highest antiproliferative activity on Au937 and Daudi cells but do not compete well for IFN-alpha2b binding sites. In order to further study species O, we are using IFN-alpha2 cDNA and species O specific oligonucleotides as probes to screen the Namalwa cDNA library. Once the cDNA clones coding IFN-alpha species O are found, restriction map and sequence analysis will be used to characterize the positive clones. Computer analysis of the sequence data may provide insight into the structural base of functional differences among these species. The cDNA for species O will be recloned into an appropriate expression vector and the expressed proteins will be purified and characterized.